It is known to prepare stable, aqueous dispersions of synthetic resins by emulsifying corresponding monomers or oligomers in an aqueous medium, using appropriate dispersants, such as emulsifiers and surfactants, and then carrying out a polymerization reaction. In this way it is possible, for example, to prepare aqueous dispersions of acrylate resins by emulsion polymerization.
In the case of polycondensation products, such as epoxy resins, which are difficult or even impossible to prepare by emulsion condensation, it is necessary, however, to prepare aqueous dispersions by dispersing the resin in water. So-called secondary dispersions of this kind are generally highly unstable, separate after only a short time, and in most cases also display poor film-forming properties.
According to EP-B 0 081 163 corresponding to U.S. Pat. No. 4,415,682, polyalkylene glycol derivatives are employed as nonionic dispersants to form stable, aqueous epoxy resin dispersions. Average particle sizes of below 1 .mu.m can be achieved. The coatings obtained with these dispersions, however, are unsatisfactory in numerous properties.
EP-A 0 051 483 corresponding to U.S. Pat. No. 4,315,044 describes epoxy resin dispersions of self-emulsifying epoxy resins which comprise polyalkylene glycol glycidyl ethers and also, if desired, a monoepoxide as reactive diluent. The maximum particle size is described as about 3 .mu.m. Films prepared from these dispersions and suitable curing agents possess, due to the content of the relatively slow-reacting polyalkylene glycol glycidyl ethers and, if desired, monoepoxides, which act as chain terminators, a relatively soft surface which restricts the utility of the epoxy dispersions.
U.S. Pat. No. 4,423,201 describes the preparation of self-emulsifying epoxy resins from diglycidyl ethers of aromatic polyols, from aromatic polyols and from reaction products of long-chain, aliphatic polyether glycols with diisocyanates and aromatic polyols. The dispersion of these resins in water, however, is possible only with the aid of relatively large quantities of organic solvents. In addition, the particle sizes obtained are relatively high. The coatings produced using these epoxy resin dispersions, moreover, are relatively soft.
German application DE-A 41 28 487 corresponding to U.S. Pat. No. 5,236,974 describes a process for the preparation of aqueous epoxy resin dispersions which comprise, as epoxy resin, a condensation product of aromatic and aliphatic polyglycidyl compounds with aromatic polyols, and, as dispersant, a condensation product of aliphatic polyols with epoxide compounds. These dispersions exhibit excellent shear stability and storage stability and are outstandingly suitable for cold-curing coatings, especially for corrosion prevention and coating applications.
German application DE-A 43 09 639 corresponding to U.S. Pat. No. 5,424,340 describes systems of analogous composition which are intended for use in heat-curing coating systems, especially for the interior coating of tins for food.
A further development of the two latter unpublished applications is seen in the preparation of absolutely solvent-free epoxy resin dispersions by the so-called monomer process. German patent application DE-A 43 27 493 corresponding to CA 2.125,254 describes the process and the preparation of such dispersions, which are in the form of epoxide-addition polymer hybrid dispersions and have further technical advantages in comparison with pure epoxy resin dispersions, such as their freedom from solvent, higher stability of the coatings, and greater possibility of variation via the choice of the monomers.
Since the solids present in these dispersions represents physical mixtures, produced in situ, of epoxy resin and polymer, and since the choice and, in particular, the quantity of the monomers is restricted by the compatibility of the corresponding polymer with the epoxy resin, there is a need to find a manner of improving the compatibility between epoxide component and polymer component. Particularly good connection of the two components in the coating can be achieved with the abovementioned dispersions if, by appropriate choice of the functional groups of the polymer, co-crosslinking is able to take place during the curing reaction.
However, it would be desirable, by appropriate modification of the epoxy resin, for the compatibility with the addition polymer to be improved even prior to and independently of the curing reaction, thereby enabling a distinct increase in the content of relatively inexpensive polymer component.